The present invention relates to the general field of odorizing natural gas, and more precisely it relates to a system and a method of injecting liquid odorant into a natural gas pipe.
Natural gas is odorless. Because of its potentially dangerous nature, present-day regulations require an odorant to be added in natural gas pipes in order to enable natural gas to be detected by means of its odor. This operation is generally performed using pure odorants or mixtures of odorants such as tetrahydrothiophene (designated by the acronym THT) or tert-butyl mercaptan (designated by the acronym TBM).
Systems for injecting odorant in liquid form into a natural gas pipe are generally dimensioned so as to be effective at the maximum observable gas flow rate at the point of injection. Nevertheless, when the real flow rate of gas becomes lower than the maximum flow rate, prior art systems for injecting odorant become less effective, which can lead to defective odorization of the gas.
Furthermore, such observed variations in the gas flow rate in pipes are particularly large when the maximum flow rate of gas to be odorized is small, as can occur in particular at points for injecting biomethane or at gas distribution stations. In addition, the opening up of gas markets to competition has led to ever increasing variability being observed in the amplitude and the frequency of the gas flow rates that can be observed, even at points for interconnecting large gas transport networks.
Various systems are known for odorizing natural gas. In particular there exist systems for injection by evaporation in which a portion of the gas for odorizing is diverted from the main flow and is put into contact with the liquid odorant, which it evaporates until thermodynamic equilibrium is reached. The diverted flow is then mixed with the main gas flow in order to obtain a mixture containing the desired odorant content.
Such evaporation systems require the supply of liquid odorant to be maintained at the same pressure as the gas flowing in the pipe, which can raise manifest problems with regulations. In addition, contact between the odorant and natural gas leads to the odorant being polluted, with it being possible for compounds in the gas to become dissolved in the odorant, thereby degrading its quality. Finally, the physical principle on which such systems are based leads to great variability in the contents of odorant in the gas if there is a change in ambient temperature (since saturated vapor pressure is a function of temperature). This physical principle is also very poorly adapted to using odorants that are made up of a mixture of chemicals, such as in particular TBM.
Another known system is that of systems using injection and a pump, in which liquid odorant is injected directly into the gas pipe by means of a diaphragm pump or by injecting odorant by means of gas under pressure. The liquid odorant evaporates in the gas by having recourse to an injection tube including a porous material, or after spraying coarse droplets.
Those injection and pump systems inject a fixed quantity of odorant each time the pump is activated. In particular, when the flow rate of gas in the pipe becomes very low, the frequency at which the pump is activated decreases, thereby leading to the system operating discontinuously. Unfortunately, the absence of back pressure between two successive actuations of the pump leads to the pump losing its priming if there is the slightest sealing defect in the pump. Furthermore, injecting a large quantity of odorant into a very low gas flow rate each time the pump is actuated leads to poor evaporation of the odorant.